1. Field of the Invention
The present invention relates to an ink-jet recording method that comprises causing flying ink (recording liquid) droplets to adhere onto recording media, thereby forming color images. More particularly, the invention relates to an ink-jet recording method using inks having different dye concentrations for recording full-color images with a wide variety of color reproduction and with good gradation.
2. Description of the Prior Art
Ink-jet recording method comprises (i) producing ink droplets by a technique for discharging inks, such as the electrostatic attraction technique based on the application of high voltage, the technique of causing mechanical vibration or displacement of the ink by means of piezoelectric elements, or the technique of heating the ink to foam instantaneously and utilizing the foaming pressure, and (ii) causing the ink droplets to adhere to recording media such as paper. This recording method is noticed in that it generates limited noises and permits high speed recording.
When color images delicate in shade or tinct like those in silver salt photographs are formed by ink-jet recording, the density gradations of various colors in the images are expressed in the conventional ink-jet recording as follows: Using inks of colors, e.g. cyan, magenta, yellow, and black, the diameter and number (per unit area) of the ink dots to be formed on recording media are controlled for color mixing, according to the densities of three primary color components in the original images to be recorded.
However, according to this method of recording with varying the diameter and number of ink dots, it is impossible to express such gradations that image density gradually varies over the entire area of from the lowest to the highest density according to original images but to reproduce low resolution images or unnatural images.
Various ink-jet recording techniques have been proposed to solve these problems. Among these, techniques of recording by use of at least two inks of different dye concentrations for each color are known from Japanese patent application Kokai No. 156264/82 and other literature.
In particular, the ink-jet recording technique disclosed in the above Japanese patent application Kokai is characterized in that; a plurality of inks of different dye concentrations are used for each of colors, the diameter and number of the ink dots to be formed are varied according to the density levels in the original image, and at the same time dots of one of the inks lower in dye concentration are mixed over almost the entire area of from low to high density of the image to be recorded. According to this technique, the density gradations of each color can be better expressed, particularly at lower image density areas, than according to the conventional technique.
In such recording techniques, however, ink properties such as viscosity, surface tension, and pH are considerably varied from ink to ink with the difference of dye concentration among plural inks, since the inks of the same color and of different dye concentrations are prepared by dissolving a dye in different concentrations in the same composition of solvents.
In particular, pH is a factor affecting the color of ink, and when one and the same dye is used at different pH conditions, the resulting color of the ink is different from each other. Therefore, when a single dye is used to produce a plurality of inks having different dye concentrations, the fluctuation of pH among the inks results in the fluctuation of color among the inks. Such inks produce images with a very unnatural feeling.